U.S. Secretary of State Condoleezza Rice says Washington has warned Iran against supporting militants in southern Iraq.

Secretary Rice said in London Sunday that Washington has tried to get the message to Iran through its rarely-used diplomatic channels. The United States and Iran have no diplomatic ties.

She said Washington uses these channels only when it wants to send stern messages.

Britain has accused Iran of backing insurgents responsible for a series of deadly attacks on coalition forces in southern Iraq. Tehran denies the charge.

London was the last stop on Ms. Rice's tour of Europe, Russia, and Central Asia. She gave her backing to British efforts to convince Iran to scrap its nuclear program. Washington believes Iran will try to build nuclear weapons.

Secretary Rice also said she believes the Iraqi constitution probably passed, calling voter turnout a blow to terrorists.